[KEP] Add `leave` statement for expressionless return

[AuthorOfTheSurf]

Currently return statements are required to have an expression, leading to suggestions of returning 0 or null when there is nothing to return. This is at odds with JavaScript and the notion of returning from a function early.

My proposal is to maintain the current behavior of return and add a leave statement that serves as the return-without-a-value. Will parse nicely and leave no doubt in the programmers mind as to what is happening where e.g.:

function double(n):
    return
    n * 2
end

seems to have undefined behavior where it is unclear if that return will grab the n * 2 on the next line (it will.)

[ebramanti]

👍 I think leave would be a useful statement in this case. In the case above, the return actually should throw an error though. Since we don't use semicolons, it should look for a return statement in the token on the current line, correct? I could be wrong tho...

Another thing to consider is Javascript functions don't necessarily require a return, unless you want to exit a function early. Which is why leave makes sense!

[AuthorOfTheSurf]

Thanks for the thumbs, I think that leave makes a lot of sense semantically. To throw the error, I will add a notion of line to tokens—so far this is the first use-case for it, but it will be available in the future.